The two basic types of optical fiber cables presently used in the industry are metallic cables and dielectric (or non-metallic) cables. At present, it is believed that slightly less than one-third of telecommunication systems use dielectric fiber optic cable. However, approximately half of those using metallic cables have indicated through surveys that they would use dielectric cables instead if there existed a reliable and cost-effective method to locate the dielectric cable after it has been buried.
Presently, there are basically two general types of technologies involved in the detection of buried fiber optic cable, magnetic and metallic. In general, the existing magnetic locators consist of either 1) magnetic elements designed or embedded into the cable, or 2) a magnetic-field emitting product which is buried alongside the length of the cable. The residual magnetization generated by the past magnetic hysteresis of the cable may then be detected by a magnetic locator. However, the distribution pattern of the magnetic field often varies according to the cable and its particular position along the cable and thus, it is difficult to detect the correct cable when other magnetic members are located nearby.
Alternatively, existing metallic fiber optic cable locators generally use a detection method based on the electromagnetic field produced by the application of an alternating current to the metallic sheathed cable. However, the electric and/or magnetic field generated by the application of an AC electric signal to the cable is often not strong enough to allow a determination of the precise location of the cable. Such low levels of field strength are particularly ineffective in locating cables buried deep under the ground or sea bed.
Commonly, to facilitate detecting a dielectric cable, a copper ground wire is positioned just above the cable. However, the exposed nature of this ground wire makes it very vulnerable to lightning strikes. Furthermore, when a cable's armor or detection wire is severed by lightning or some other cause, it becomes impossible to apply an electric signal along the cable, thus creating great difficulty in locating and retrieving the cable for repair.
Other existing detection methods include incorporating a non-conductive tape which has been covered with a magnetic powder such as strontium or barium ferrite or compounding and extruding these magnetic powders in polyethylene or polyvinyl chloride conduits. Both of these methods employ a means of magnetizing a tape along its width, and helically wrapping it along the length of the cable or of the conduit. Each method also provides a distinctive electronic-detection signature which allows an operator to differentiate between a buried cable and a solid metallic pipe. See U.S. Pat. Nos. 5,006,806 and 5,017,873 which are assigned to Schondstedt Instrumentation Company of Reston, Va.
What is needed and seemingly not available in the prior art is a system which dependably, accurately and cost-effectively locates dielectric (non-metallic) buried cables. Also desirable is a system for locating buried dielectric cables which is readily adaptable to most, if not all, existing cable types.
One method involves modifying the existing water-blocking tape present in the cable so that the cable becomes magnetically locatable without adversely affecting the operational characteristics of the cable. See copending and commonly-assigned U.S. Ser. No. 08/012357 now allowed. However, while addressing a concern similar to that of the above-identified copending application, the present invention is directed toward modifying the strength member portion present in most communication cables instead of the waterblocking portion as taught by the application identified immediately above.